Detailed Description

template<typename T>
struct identity< T >

A template class that simply exports its template argument as a local alias. This class, while at first appearing useless, makes sense in the following context: if you have a function template as follows:

template <typename T>

void f(T, T);

then it can't be called in an expression like f(1, 3.141) because the type T of the template can not be deduced in a unique way from the types of the arguments. However, if the template is written as

template <typename T>

void f(T, typename identity<T>::type);

then the call becomes valid: the type T is not deducible from the second argument to the function, so only the first argument participates in template type resolution.

The context for this feature is as follows: consider

template <typename RT, typename A>

void forward_call(RT (*p) (A), A a)

{

p(a);

}

void h (double);

void g()

{

forward_call(&h, 1);

}

This code fails to compile because the compiler can't decide whether the template type A should be double (from the signature of the function given as first argument to forward_call, or int because the expression 1 has that type. Of course, what we would like the compiler to do is simply cast the 1 to double. We can achieve this by writing the code as follows: